Conventional resistance spot welding techniques employ a method by which metal surfaces are joined together in one or more spots by the heat generated by the resistance to the flow of electric current through the workpieces that are held together under force by the electrodes. The contacting surfaces in the region of current concentration are heated by a pulse of high amperage current to form what is known as a weld nugget at the interface between the two surfaces. Ordinarily, when the flow of current ceases, the electrode forces are maintained for a while to allow the weld nugget to cool and solidify. An excellent discussion of the details of the metalurgical phenomena that occurs during resistance spot welding is found in Nied, "The Finite Element Modeling of the Resistance Spot Welding Process", Welding Research Supplement, pages 123-132 (April, 1984).
The popularity of resistance spot welding is due in large part to its capability of rapidly producing welds with apparatus that can be used in high volume, automated production. For example, the recent trend is for vehicle manufacturers to use automatic machinery employing resistance spot welding guns attached to the ends of robot arm actuators to automatically position the gun and control the welding cycle. While resistance spot welding has these and many other advantages, it is unfortunately somewhat difficult to control the process satisfactorily in order to produce consistently good welds. Many different factors must be controlled such as voltage, current, pressure, heat loss, shunting, and electrode wear, as well as the thickness and composition of the workpiece material. Many of these variables are difficult to consistently control.
Several attempts have been made to automatically control resistance spot welding processes. For example, some techniques are designed to regulate the amount of energy used during the welding cycle. To this end, current sensors and voltage regulators have been incorporated in feedback systems which compare the detected levels with certain preset references. These feedback systems are disadvantageous from the standpoint that they do not directly detect physical characteristics of the weld itself but instead rely upon detection of secondary parameters. In other words, they detect the means and not the end result.
Other techniques provide some means for determining whether the metal of the workpieces have reached a molten state. If the metals to be welded do not reach the temperature required to become molten, an insufficient weld or no weld at all will result. It has been shown through measurements that as soon as the molten state is reached, the electrodes, which are being forced against the workpiece, begin to move into the metal and toward each other. Accordingly, it has been suggested that the detection of melting by sensing subsequent inward movement of the electrodes (indentation) or penetration is a potentially good way of determining the state of the weld. However, just because the metal reaches a molten state does not always insure that a good weld is made. For example, too much weld current will produce melting but not necessarily the formation of the weld nugget which is an important factor in generating a good weld. Still other parameters will effect the size and configuration of the weld nugget and the prior techniques of merely sensing inward movement of the electrodes into the workpieces cannot readily determine the extent of weld nugget growth. U.S. Pat. No. 4,419,558 to Stiebel accomplishes the detection of electrode travel indirectly by utilizing a load cell to monitor the squeezing force applied through the electrodes to the workpieces. Among the disadvantages with this construction is that its sensor is located very close to the position at which the weld is made and it does not lend itself to incorporation into many welding gun designs, especially self-equalizing guns that are connected to robot arm actuators. Apparently other attempts have been made to use detection of electrode movement to control the welding process but they have been unsuccessful because of the difficulty of measuring in an accurate and repeatable way the small distances involved in the travel of resistance spot welding electrodes, which are of the order of 0.001 inch.
Improved resistance spot welding apparatus is provided according to the present invention for automatically and consistently reliably detecting the quality of resistance spot welds. The apparatus includes a cylinder with two opposite ends and a piston assembly within the cylinder. The piston assembly has a piston rod extending through both of the ends of the cylinder. One end of the piston rod can be coupled to an electrode for making the weld and the opposite end of the piston rod can be coupled to a sensor asembly for detecting the quality of the resulting weld as a function of movement of the piston rod. The apparatus lends itself to a compact and relatively simple design that can be easily incorporated as part of a variety of resistance spot welding gun designs.
In the preferred embodiment, the sensor assembly is attached to the rear of the cylinder and includes a shaft attached at one end to the rearward end of the piston rod extending through the cylinder. A transversely mounted plate on the end of the shaft cooperates with a transducer mounted within the assembly to measure movement of the piston rod during the welding cycle. Since the piston rod is directly coupled to the electrode, extremely accurate measurements of electrode movement can be obtained, even though the sensor assembly is located some distance away from the electrode. As a result, the sensor assembly will not interfere with the welding operation and its measurement is less likely to be affected by the electromagnetic forces created at the electrode tips.
According to the method of this invention, the maximum forward position that the electrode reaches during the welding process is detected and subtracted from the distance that the electrode retracts from that maximum position to thereby provide an indication of the extent of weld nugget growth. The measurement of weld nugget growth can be used as a parameter in determining the quality of the weld and/or unfavorable trends of the welding operation.